[Impeller] dont call SPIRV_CROSS_THROW in SkSL backend

[jonahwilliams]

SPIRV_CROSS_THROW can be configured to either throw an exception or print a message and abort. We don't enable exceptions, so the latter is the only choice. For now, though this has two additional problems:

• The message is only printed in unopt/debug builds
• The abort is caught by our own exception handler in fml/backtrace.h which adds a nasty stack trace.

Long-term, we might consider sending a patch to change this behavior. In the short term, we can improve the messages we add by using a differen macro

Before:

$impellerc --sksl --iplr --sl=shaders/hello.glsl --spirv=shaders/hello.glsl.spirv --input=shaders/hello.glsl --input-type=frag --include=examples/hello_world/shaders
[ERROR:flutter/fml/backtrace.cc(108)] Caught signal SIGABRT during program execution.
Frame 0: 0x102fce3b8 spirv_cross::report_and_abort()
Frame 1: 0x102fd5b67 impeller::compiler::CompilerSkSL::detect_unsupported_resources()
Frame 2: 0x102fd486c impeller::compiler::CompilerSkSL::emit_resources()
Frame 3: 0x102fd42da impeller::compiler::CompilerSkSL::compile()
Frame 4: 0x102f8a741 impeller::compiler::Compiler::Compiler()
Frame 5: 0x102f87092 impeller::compiler::Main()
Frame 6: 0x102f887d8 main
Frame 7: 0x20413d52e start
Frame 8: 0x3 0x0

After:

$impellerc --sksl --iplr --sl=shaders/hello.glsl --spirv=shaders/hello.glsl.spirv --input=shaders/hello.glsl --input-type=frag --include=examples/hello_world/shaders
There was a compiler error: SkSL does not support UBOs or SSBOs: 'data'

Fixes flutter/flutter#114550
Work towards flutter/flutter#114536

[flutter-dashboard]

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[jonahwilliams]

can we invoke impellerc in a unit test? It would be nice to have a simple verification of a specific error/shader combo

[dnfield]

can we invoke impellerc in a unit test? It would be nice to have a simple verification of a specific error/shader combo

You could add something to run_tests.py that calls it and verifies output/exit code.